Method of inkjet printing for use in point-of-sale systems

ABSTRACT

This invention relates to a method of inkjet printing, in particular for use in POS systems. In particular the printing method according to the invention is of the thermal type. In order to obtain high speed printing, for example of 15-20 lines per second when printing text on a paper medium 60-80 mm wide (for example receipts), the drops of ink ejected during the printing operation advantageously have a relatively large volume, i.e. not less than approximately 100 picoliters (pl) and preferably between 100 and 200 pl. The ink used for applications in POS systems must quickly penetrate an extensive range of paper media. This requirement is satisfied by using an ink having a surface tension of not more than approximately 35 dyne/cm (mN/m). Problems of stagnation and dripping of the ink caused by the printing conditions are overcome through the use of a nozzle plate coated with a wetting-resistant layer of silicon carbide. This coating material has proved to have non-wettability properties which do not significantly deteriorate during a printing operation over a long period of time.

TECHNOLOGICAL FIELD OF THE INVENTION

This invention relates to a method of inkjet printing and in particularrelates to a method of printing for use in a system for issuingreceipts, tickets or bank certificates at points of sale (POS) oroffices.

STATE OF THE ART

In applications in the retail sales business, post offices or bankssector many types of printing devices are currently in use to issuereceipts, tickets or bank cheques. These printing devices may beconnected to or incorporated into terminals for lottery games, cashregisters or bank validators. Systems for issuing tickets, receipts orbank cheques for commercial points of sale and/or public offices andwhich include printing devices will be referred to below as point ofsale (POS) systems.

Printing devices for POS systems generally operate by printing data ontoa continuous strip of paper originating from a roll on which the stripis wound, and subsequently cut off the printed strip in such a way as toform the receipt. The technologies most frequently used for printingreceipts are the so-called dot matrix impact printing technology andinkjet technology. In the first case printing is typically effectedusing a printing head of the type having points which are selectivelyheated, and during printing the head is in writing contact with aspecial paper known as thermal paper. In the second case printing iseffected through a printing head comprising a plurality of nozzlescapable of selectively emitting drops of black or coloured ink onto thepaper while the head moves alternately (forwards and backwards) andtransversely with respect to the driven movement of the paper. In thecase of inkjet printers of the thermal type the head uses heatingelements, generally resistors, which heat the ink in order to boil itand therefore cause the ink to be expelled through the nozzles duringthe printing operation. The paper normally used for inkjet printing isof the ordinary type. The possibility of using ordinary paper rendersinkjet technology particularly advantageous because it is relativelyeconomical, especially when printing in black and white.

In applications for POS systems it is important that the stagesassociated with printing and the issue of a receipt are carried outquickly, in order to reduce user waiting times, especially in placessuch as supermarkets and gaming and lottery offices. The need to havehigh printing speeds, often combined with the need to keep the cost ofprinting low, means that resolutions which are not particularly high,for example between 100 and 200 dpi, are generally acceptable.Typically, also, again to keep printing costs low, the printers useblack ink, although colour printers are beginning to come intowidespread use, for example for the printing of logos on tickets.

In addition to this the ink used in POS applications must dry almostinstantaneously because when the printing of receipts is completed thelatter are immediately placed into the hands of the customer, whosefingers must not be soiled. An ink for POS printers which has a dryingtime of the order of 300 ms or less is known for example from patentapplication EP 1142967.

The operation of a printer for POS terminals must often be capable ofmaintaining the printing condition for long periods of time, for examplefor several hours or continuous printing of thousands of tickets withoutintervention by the operator. Continuity of operation has resulted inprinters provided with high capacity reservoirs, for example containing200 cc of ink, being developed. Various systems for supplying ink orinks (for example in the case of colour printing where inks of differentcolours are used) to a cartridge for an inkjet printing head are knownin the state of the art. Patent application WO 2004/091918 discloses adevice for simultaneously supplying inks of different colours to acolour cartridge of a printing head which comprises a container withinwhich there is located a seat for the cartridge which has to be reloadedand at least three reservoirs for different coloured inks.

The printer may be provided with a high capacity ink reservoir on boardthe machine. Patent application WO 03/097362 describes an inkjet printerused at points of sale in which the service or housing position for theprinting head coincides with the position in which ink is supplied tothe cartridge, which is of one piece with the head. The cartridgeintegral with the head is supplied from a separate ink reservoir mountedon the structure of the printer, to which the structure is connected atintervals governed by measurement of the level of ink present therein.

In order to clean and protect the printing head, printers may beprovided with a cleaning station (also known as a “service station”)capable of removing residues of ink and located along the trajectory ofthe head, typically on board the machine and at one extremity of thehead's travel in a resting position. The cleaning element at thecleaning station is generally a flexible rubber blade which wipes theouter surface of the head from which the ink drops are emitted. Anexample of a service station for the cleaning of an inkjet printingmechanism is described in U.S. Pat. No. 6,527,362.

Patent application WO 01/39979 describes a small POS inkjet printer andin particular equipment for periodically cleaning the nozzles of one ormore cartridges which are moved reciprocally through the machine'sprinting station.

Both abrasion and deformation of the nozzle plate can occur duringcontact between the head and the other structures encountered in theprinting operation, such as cleaning structures. The problem of thedurability of the head is particularly present in the case of nozzleplates made of non-metal polymer material. Patent application EP 1306215describes a coating layer on at least one of the upper or lower surfacesof a nozzle plate to render the head more robust. Coating materials suchas silicon nitride (Si₃N₄), boron nitride (BN), silicon oxide (SiO₂),silicon carbide (SiC) and a composition known as “silicon carbon oxide”are used for this purpose.

As previously mentioned, the ink is ejected from a printing head througha nozzle (orifice) formed through a nozzle plate. The build-up ofmaterial at the nozzle may affect formation of the drop, attract dust orother micro-debris, and may also cause smearing of the ink. For thisreason it may be desirable that the surface of the nozzle plate shouldhave a low wettability (being non-wetting or anti-wetting) with respectto the fluid ejected through the nozzle.

U.S. Pat. No. 6,610,165 describes a method for coating a nozzle platewith a non-wetting Teflon (PFTE) material formed by thermal compression.

SUMMARY OF THE INVENTION

This invention relates to a method of inkjet printing, in particular foruse in POS systems. In particular the method of printing according tothis invention is of the thermal type.

The Applicant has noted that prolonged operation in the printingcondition, such as in the case of POS applications, may result in thedegradation of print quality due to partial or total obstruction of thenozzles, with the consequent loss of printing points on the paper medium(“missing dots”), or loss of direction in ejection of the drop.

The Applicant has observed that in order to obtain high speed printing,for example of 15-20 lines per second for a printing head on a papermedium 60-80 mm wide (e.g. receipts), the drops of ink ejected duringthe printing operation must advantageously have a relatively largevolume, that is not less than approximately 100 picoliters (1 pl =10⁻¹²liters), and preferably between 100 and 200 pl.

Specific requirements associated with the use of POS systems, forexample the possibility of reading the receipt while it is being printedin a cash register or during the validation of a cheque in a banksystem, have often made it necessary to use specific paper passes acrossthe width of the paper medium with the head horizontally positioned,that is with the rows of nozzles substantially perpendicular to thedirection of the width of the paper medium (transverse motion of thehead) and substantially parallel to the printing direction (feeddirection for the paper medium).

The Applicant has noted that a horizontal position of the head withrespect to the paper medium presents problems of ink dripping onto theelectrical contacts of the head, with consequent problems of anelectrical nature, such as short circuits. In particular the presence ofstagnating ink which builds up over time, particularly at the bottom ofthe head (with respect to the printing direction), because the force ofgravity causes the ink to flow, has been observed. It has been foundthat dripping may occur for example after 20-30 cc of ink have passedthrough the nozzles of the printing head, corresponding to a relativelyshort operating time for the printer, given that a printer for POSterminals generally uses high printing speeds and must be capable ofmaintaining the printing condition for a long period of time.

Although these disadvantages can in part be overcome through actions toclean the outer surface of the head (i.e., of the nozzle plate), theApplicant has noted that in the case of continuous printing it isnecessary to activate cleaning actions frequently, for example throughfrequent passages of the head past the service station at the edge ofthe machine. This may however result in damage to the head due to theabrasive action between the cleaning element and the nozzle plate, and areduction in productive potential (throughput) because cleaning timeobviously detracts from printing time.

The Applicant has also found that the need for frequent cleaning of theprinting head shifts the problem of dripping to the head cleaningelement, typically a flexible blade. In fact, it has been found that thecleaning element has dripping problems after frequent cleaning (carriedout for example every 0.1 cc of ink emitted from the nozzles) after aprinting cycle corresponding to the use of a total volume ofapproximately 60 cc of ink. The ink dripping from the bottom of thecleaning element wets the carriage transporting the printing unit duringprinting, with consequent damage to the electrical contacts of theprinting unit and the carriage.

If the ink drops are ejected in relatively large volumes it has beenobserved that the problems of the build-up of residues and/or drippingof the ink become more severe, because these phenomena occur in arelatively short time, with a consequent need for even more frequentcleaning actions.

The ink used in printing for POS systems must have the properties ofquickly drying and/or quickly penetrating the paper so that the printedpaper, in the case of a receipt or a lottery game ticket for example,may be immediately handed over to the customer. The Applicant has foundthat the requirement of having an ink offering such performance on avast range of paper media, such as watermarked paper, cheques andpreprinted tickets, is satisfied by using an ink having a surfacetension of not more than approximately 35 dyne/cm (mN/m), as measuredusing a Krüss K12 digital tensiometer. In particular the surface tensionis determined at ambient temperature by the maximum value of the forceat the contact moment between the sample under test and a platinum plate(Wilhelmy plate). Preferably the surface tension of the ink is betweenapproximately 25 and 35 dyne/cm.

The ink is preferably a black ink.

Low surface tensions (for example around 30 dyne/cm) increase the“wettability” of the ink on the surface of the nozzle plate, that is thelower the surface tension of the liquid the more the ejected drop willtend to spread over the surface, that is to form a smaller angle ofcontact with the surface. It has therefore been found that the use of anink with a low surface tension combined with the need to print byemitting drops of relatively large volume has rendered the problem ofwetting of the head and therefore that of dripping of the inkparticularly unacceptable. If the situation is already compromised, forexample if a film of ink has formed on the outer surface of the plate,it has been observed that cleaning action can encourage furtherspreading of the ink onto the surface of the nozzle plate instead ofproviding benefit.

The Applicant has considered that if the surface of the nozzle platethrough which the ink drops are ejected (i.e. the ejection surface),that is the surface with which the drops come into contact, issufficiently wetting-resistant (anti-wetting), the drops will spread toa lesser extent.

Teflon (or Teflon-like materials, CF_(x)) which is an organic materialhaving wetting-resistant properties, has been considered for thispurpose. The nozzle plate considered was a nozzle plate of nickel coatedwith gold. Because it was noticed that Teflon did not adhere well togold; the upper gold surface of the nozzle plate was coated with a layerof silicon carbide (SiC) acting as an adhesive for the organic material.

In particular the silicon carbide was deposited using typicaltechnologies for semiconductor processes, such as plasma-enhancedchemical vapour deposition (PECVD). A layer of Teflon-like material(CF_(x)) was formed through a plasma polymerisation process.

The Applicant has observed that although the Teflon-like material hadsatisfactory wetting-resistance properties at, the start of the printingcycle, these properties nevertheless substantially deteriorated in thecourse of the printing operation, for example after a printing cycleequal to the ejection of a total volume of approximately 20 cc of inkfrom the nozzles (i.e. the total volume of ink which passed through thenozzles in the head). In addition to this, the Teflon showed a tendencyto be detached from the underlying surface, even in the case where theunderlying surface was of SiC.

It was specifically when examining the wetting-resistance properties ofthe ejection surface for the ink drops following detachment of theTeflon layer from the ejection surface of the head that the Applicantfound that the layer of SiC used as an adhesive material in fact hadvery satisfactory wetting-resistant properties for the purposes of thisinvention. Significantly, after a printing cycle equal to the ejectionof approximately 20 cc of ink, this wetting-resistance property of theSiC did not deteriorate significantly, but instead remained quiteconstant. The lack of deterioration in the wetting-resistance propertyof the SiC coating is a feature that makes it possible for the head tobe used for POS applications without incurring problems associated withdripping of the ink during the printing operation.

The experiments performed by the Applicant are reported in greaterdetail below.

The property of the wettability (or non-wettability) of the surface ofthe nozzle plate may be evaluated by measuring the contact angle, α,between a drop of ink and the surface of the nozzle plate. FIG. 4illustrates schematically the formation of a drop 23 on an upper surface26 of a nozzle plate 28. Angle α corresponds substantially to the anglewhich the tangent 24 to the surface of the drop forms with the plane ofthe upper surface of the head, 26. The greater the value of α, the morethe spreading of the drop is restricted, and the drop has well-definedperimeters. In other words, the higher the value of α, the more the dropis in contact with a less-wettable surface (for the same surface tensionof the fluid forming the drop). Measurements of the contact angle werecarried out at ambient temperature (22-25°) using a commercial OCA 20static angle measuring system distributed by FKV, depositing a drop ofliquid on the surface of a nozzle plate using a micropipette.

The most significant results of the experimental tests are shown inTable 1. Table 1 shows the contact angle resulting from measurementsmade on a drop of liquid having a volume of 0.6 μl (6.0×10⁻⁷ l)deposited on the surface of the materials indicated in the table. Twomeasurements of the contact angle were made for the ink—one immediatelyafter deposition of the drop (α₀) and a second measurement after a setof printing operations corresponding to the use of a total volume ofapproximately 20 cc of ink (α₁). This set of printing operationscorresponds to approximately one life cycle of a disposable head. By wayof reference Table 1 also shows measurements relating to the initialcontact angle (α₀) of a drop of water, which was always 0.6 μl. A thirdcontact angle measurement (α₂) was made following an immersion testconsisting of immersing the nozzle plate in ink for a period of sevenweeks at a temperature of 65° C. The ink used in the tests had a surfacetension of approximately 30 dyne/cm at 25° C. By way of reference thesurface tension of H₂O at 25° C. is 71.9 dyne/cm. The dependence of thecontact angle on the surface tension of the fluid, for the same surfacematerial of the nozzle plate, will be noted from the values of α₀ shownin Table 1.

TABLE 1 Coating material of the surface of the α₀ α₀ α₁ α₂ nozzle plateH₂O Ink Ink Ink Au 80-90° 40-50° <25° 30° SiC  90-100° 50-60° >45° 50°Teflon 100-110° 50-60° *  35°** *Not determinable because of detachmentof the Teflon layer **The test was interrupted after 3 weeks because ofdetachment of the Teflon layer.

The experimental tests show that the initial contact angle in the caseof the Au-coated surface is less than that of the SiC-coated surface.Also, in the case of the Au surface, net deterioration of thenon-wettability of the surface occurred, corresponding to a reduction ofat least a factor of 2 in the contact angle. In the case of SiC on theother hand the contact angle remained virtually constant, or at leastthe reduction in the contact angle was not greater than 20-25%.

Teflon has not shown good adhesion to Au metal surfaces or SiC, thelatter being used as a material to promote adhesion. However preliminaryresults of the immersion test (α₂) appear to suggest that the reductionin the contact angle over time will be greater than that observed in thecase of SiC.

The Applicant, has found that a wetting-resistant surface coating ofsilicon carbide on the upper surface of the nozzle plate ensures thatthe nozzle plate has stable non-wettability properties in the course ofthe printing operation.

Preferably the contact angle α₁ of the wetting-resistant layer will notbe less than approximately 45°.

The Applicant has observed that a surface characterised by anexcessively large contact angle α₁ may result in adhesion problems withany layers above the nozzle head, such as for example the tape sealingthe nozzles which must adhere to the plate until it is first used by theend customer. Preferably the contact angle α₁ is not greater thanapproximately 90°.

When a SiC coating is present on the upper surface of the nozzle plateit has been observed that the drops remain close to the holes, and as aresult of transitory hydraulics following ejection, are partly drawnback within the nozzle, with consequently less ink on the surface of thenozzle plate.

The nozzle plate obtained has the advantage of reducing the number ofcleaning operations necessary in order to continue the printingoperation, with consequent extension of the service life of the head.Also, if the surface of the plate has a wetting-resistant SiC coating,cleaning operations have a positive effect in removing printing residueswithout risking deterioration of the quality of printing subsequent tothat operation.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a schematical view of a cartridge with an inkjet head usedin the method of printing according to a preferred embodiment of theinvention.

FIG. 2 is a partial schematical cross-section through a printing headaccording to a preferred embodiment of the invention.

FIG. 3 shows a schematical perspective view of a printing head accordingto a preferred embodiment of the invention.

FIG. 4 is a schematical representation of the contact angle of a drop onthe ejection surface of a nozzle plate.

DETAILED DESCRIPTION

In the preferred embodiment of the invention the method of printing usesan inkjet printing head of the “top shooter” thermal type, that is onewhich emits ink drops in a direction substantially perpendicular to theejection members. This head is manufactured using semiconductor wafersand processing technology typical of the semiconductor componentsindustry.

FIG. 1 illustrates a printing unit 100 comprising an inkjet printinghead 10 according to the invention combined in one piece with an inkfeed cartridge (not shown) of the rechargeable type and containing aspongy body. Printing unit 100 is provided with electrical contacts 102to connect the cartridge to the printer (not shown). This cartridge isheld in a container 103 and is closed by a lid 101. The cartridge may besupplied with ink by transferring ink from a main high capacityreservoir (not shown) through recharging hole 104. For example a mainhigh capacity reservoir (e.g. for 300 cc of ink) may be mounted on thestructure of the printer and printing unit 100 may be connected to themain reservoir in order to be recharged with ink at predeterminedintervals or at intervals depending upon the level of ink determined bya sensor in accordance with known methods. For example the cartridge ofthe printing unit may be capable of containing up to 20 cc of ink.

FIG. 2 illustrates a partial transverse cross-section of a printing headaccording to a preferred embodiment of the invention. Head 10 comprisesa silicon substrate 1 on which there is formed a layer of silicon oxide6. A plurality of heating elements 2 (only one element is illustrated inFIG. 2), for example resistors of Ta/Al, are deposited on silicon oxidesurface 6. A layer or a plurality of protective layers 3, for example aTa/SiC/Si₃N₄ multilayer, covers the resistors in order to protect them.

Printing head 10 comprises a plate 12 in which are provided a pluralityof nozzles 7 from which ink drops directed against the printing medium,which is generally paper (not shown), are expelled. Each nozzle 7 ispositioned in relation to a chamber 5 where a bubble of vapour formsfollowing heating of resistor 2. Nozzle plate 12 is of metal and ispreferably of gilded nickel. In FIG. 2 a galvanic nickel plate (grownfor example by electroforming) 13 is coated with a layer of galvanicgold 14 again obtained, for example, by electroforming. Preferably alayer of gold 15 and 16, respectively, having a thickness of some nm(for example 2-5 nm) is deposited by sputtering onto both the upper andlower surfaces of the Au/Ni plate. Preferably the surface of thegalvanic layer of gold 14 is treated by sputter etching using argon gasplasma in order to clean the surface before depositing Au layers 15 and16 by sputtering.

The metal plate is attached to a layer of photopolymer 4 in which areprovided chambers 5 and conduits (not shown) through which the ink flowsto the chambers from a reservoir fed by the cartridge (not shown). Inorder to improve the adhesion of the nozzle plate on photopolymer 4,gold layer 15 on the lower wall of the head is coated with a layer oftantalum (Ta) 11 subsequently coated with a layer of silicon carbide(SiC) 19. The SiC 19 acts as a promoter of adhesion between gold layer15 and photopolymer 4. Intermediate Ta layer 11 is inserted to encourageadhesion between the SiC and the gold surface. Ta layer 11 is depositedon the gold surface for example by sputtering. For example a layer of 40nm of SiC is deposited on a 30-50 nm layer of Ta.

According to the invention the injection surface of the nozzle plate,that is the surface from which the ink drops are emitted through thenozzles, is coated with a wetting-resistant SiC layer 18. Preferablywetting-resistant SiC layer 18 is not more than approximately 50 nmthick, more preferably between approximately 30 and 40 nm. Knowntechnologies such as PECVD may be used to deposit the layer of SiC.

In the preferred embodiment of the invention illustrated in FIG. 2,wetting-resistant SiC layer 18 is deposited on a Ta layer 17 formed onthe upper surface of the nozzle plate. This Ta layer encourages adhesionof the SiC, this adhesion being greater than that to a gold surface. Inthe embodiment in FIG. 2 the Ta layer is formed on Au layer 16 obtainedby sputtering. Preferably Ta layer 17 has a thickness of 30-50 nm.

FIG. 3 illustrates a perspective view of head 10 in which nozzle plate12 is covered by a wetting-resistant SiC layer 18 (hatched area in FIG.3). Preferably the SiC film is present substantially over the entiresurface of the metal plate. Known masking techniques may define the areaof deposition of SiC, avoiding for example deposition on electrical pads20, to which a flexible circuit tape (not shown) will be attached byknown Tape Automatic Bonding techniques.

The printing head according to the invention emits drops of ink having avolume of at least approximately 100 picoliters (10⁻¹⁰ l), preferablybetween 100 and 200 pl. The nozzles from which these drops are emittedare preferably circular and have a diameter of preferably between 45 and60 μm. For example the head emits drops of approximately 150 pl fromnozzles 53 μm in diameter.

If the printing resolution required is not particularly great, forexample between 100 and 200 dpi, the number of nozzles need not beparticularly large. For example 75 nozzles are arranged in twosubstantially parallel rows, preferably offset by an amount equal to1/150 of an inch. In an individual row the nozzles are arrangedsubstantially along the direction and with a spacing of 1/75 inch. Withthis nozzle arrangement a resolution of 1/150 inch is obtained.

During the printing operation the printing head is caused to movesubstantially along the x direction, i.e. the rows of nozzles arearranged substantially perpendicular to the direction of transversemovement of the head, typically in the direction of the width of thepaper medium, while the paper medium moves along y.

For example, a suitable ink composition according to the inventioncomprises:

-   -   aqueous solvent: comprising water deionised to 18 Mohm, used in        a percentage of approximately 80% w/w,    -   humectant comprising water-soluble organic solvents as sols or        as mixtures between them, such as low molecular weight glycols,        glycoethers (for example diethylene glycol-monobutyl ether),        2-pyrrolidone,    -   dye, such as Food Black 2 or Direct Black 168,    -   biocide, such as products containing isothiazolinone compounds,        and    -   surfactant: a compound belonging to the family of acetylene-diol        ethyoxylates in a concentration varying between 1 and 3% w/w.

The surface tension of the composition in the example described variesbetween approximately 32 and 35 dyne/cm.

1. Method of inkjet printing using a head comprising a nozzle plate inwhich there is formed a plurality of nozzles through which ink drops areejected, the said nozzle plate comprising an ejection surface from whichthe drops form; the said method comprising: ejecting ink drops having avolume of not less than approximately 100 picoliters, the said inkhaving a surface tension of not more than approximately 35 dyne/cm,wherein the said surface of the nozzle plate is coated by awetting-resistant layer of silicon carbide; and the ink drops form acontact angle of not less than approximately 45° with thesilicon-carbide-coated surface.
 2. Method of printing according to claim1, in which the silicon carbide layer has a thickness of not more thanapproximately 50 nm.
 3. Method of printing according to claim 1, inwhich the nozzle plate is of metal.
 4. Method of printing according toclaim 3, in which the nozzle plate is of nickel coated with gold. 5.Method of printing according to claim 4, in which the upper surface ofthe plate is coated with a layer of gold formed by sputtering.
 6. Methodaccording to claim 3, in which an intermediate layer of tantalum isformed between the upper surface of the plate and the layer of siliconcarbide.
 7. Method of printing according to claim 1, in which the volumeof the ink drops ejected is between 100 and 200 pl.
 8. Method ofprinting according to claim 1, in which the surface tension of the saidink is between 25 and 35 dyne/cm.
 9. Method of printing according toclaim 1, in which the duration of the said ink drop ejection step issuch that a total volume of at least approximately 20 cc of the said inkis ejected and in which after said ejection step the ink drops form acontact angle of not less than approximately 45° with the saidsilicon-carbide-coated ejection surface.
 10. Method of printingaccording to claim 1, in which the said contact angle following the saidejection step is between approximately 45° and approximately 90° . 11.Printing unit for inkjet printing comprising: a printing head comprisinga nozzle plate in which there is formed a plurality of nozzles fromwhich ink drops are ejected and an ejection surface from which the dropsform, and a cartridge containing ink supplied to said printing head, thesaid ink having a surface tension of not more than 35 dyne/cm, whereinsaid nozzles have an opening onto the said ejection surface of theprinting head of a diameter between approximately 45 and approximately60 μm, and wherein said ejection surface of the nozzle plate is coatedwith a layer of wetting-resistant material such that the ejected dropsform a contact angle of between approximately 45° and approximately 90°on said wetting-resistant layer.
 12. Printing unit according to claim11, in which said layer of wetting-resistant coating comprises siliconcarbide.
 13. Printing unit according to claim 12, in which the layer ofsilicon carbide has a thickness of not more than approximately 50 nm.14. Printing unit according to claim 11, in which the nozzle plate is ofnickel coated with gold.
 15. Method of inkjet printing using a headcomprising a nozzle plate in which there is formed a plurality ofnozzles through which ink drops are ejected, the said nozzle platecomprising an ejection surface from which the drops form; the saidmethod comprising: ejecting ink drops having a volume of not less thanapproximately 100 picoliters, the said ink having a surface tension ofnot more than approximately 35 dyne/cm, wherein the said surface of thenozzle plate is coated by a wetting-resistant layer of silicon carbide;the nozzle plate is of metal; and an intermediate layer of tantalum isformed between the upper surface of the plate and the layer of siliconcarbide.
 16. Method of printing according to claim 15, in which thesilicon carbide layer has a thickness of not more than approximately 50nm.
 17. Method of printing according to claim 15, in which the volume ofthe ink drops ejected is between 100 and 200 pl.
 18. Method of printingaccording to claim 15, in which the surface tension of the said ink isbetween 25 and 35 dyne/cm.
 19. Method of printing according to claim 15,in which the ink drops form a contact angle of not less thanapproximately 45° with the said silicon-carbide-coated surface. 20.Method of printing according to claim 15, in which the duration of thesaid ink drop ejection step is such that a total volume of at leastapproximately 20 cc of the said ink is ejected and in which after saidejection step the ink drops form a contact angle of not less thanapproximately 45° with the said silicon-carbide-coated ejection surface.